The present invention relates to a method of cathodically depositing polymeric coatings. More specifically, it relates to a method of depositing coatings of acrylic polymers containing amine functionality with minimum retained water and acid in the coatings.
It is known that organic coatings can be electro-deposited either on an anodically-charged conducting substrate or on a cathodically-charged substrate. Although most of the earlier work in electrodeposition was done with anodic deposition, that type of process has certain disadvantages. Anodic electrodeposition is normally done in a coating bath having a basic pH. The pH decreases at the surface being coated, creating conditions which, when combined with the electrolytic action of the coating bath, can cause the dissolution of substrate metal ions and their subsequent deposition in the coatings being formed. This can be a source of staining and diminished corrosion resistance. Also, electrolysis tends to attack preformed phosphate coatings. Furthermore, oxygen formed at the anodic substrate being coated can cause a variety of difficulties such as degradation of coatings by oxidation.
Electro-endoosmosis tends to expel water from anodic coatings being formed, leading to low water retention with about 85 to 95% solids in the coatings. This is an advantage over cathodic coating in which this phenomenon would not be expected to be helpful. (Parts and percentages herein are by weight except where indicated otherwise, and the expression of a range as "X to Y" or as "X-Y," wherein X and Y are numbers, is meant to be inclusive of both X and Y.)
Cathodic electrodeposition has developed more slowly, due in part to the acidic pH needed for the bath. Also, water tends to be drawn into the coatings and held there, along with acid residues from the bath. It is apparent that this can lead to difficulties in the coatings. In contrast to the oxygen formed at anodes in anodic electrodeposition, hydrogen is formed at the cathode in cathodic electrodeposition. Even though this hydrogen can cause pinholes in coatings, it, of course, does not cause oxidative film degradation.
Prior to coating with protective organic coatings, metal surfaces, particularly iron and steel, are normally given a pretreatment such as phosphatizing. U.S. Pat. No. 870,937 -- Coslett (1907) describes a method of treating iron or steel surfaces with phosphoric acid solutions which may include iron powder or iron phosphates. In the evolution of phosphatizing coatings for metals, particularly ferrous metals, several chemical modifications of the phosphate coating have been found desirable, including the incorporation of calcium and molybdenum into the coating and post-rinsing with chromate solutions.
Processes and compositions for the cathodic electrodeposition of paints are described in U.S. Pat. No. 2,345,543-Wohnsiedler, et al., (1944), which uses a cationic melamine-formaldehyde resin, and in U.S. Pat. No. 3,922,212-Gilchrist (1975), among others. Gilchrist is directed to a process for supplementing the bath composition with a make-up mixture of materials containing an ionizing acid that is not consumed at as fast a rate as the resin. The acid is present in the make-up at lower concentrations than are used in the bath, so as not to build up the concentration of the acid in the bath. Gilchrist uses particular aminoalcohol esters of polycarboxylic acids and discloses that acrylic polymers can be codeposited with zinc phosphate from solution on a cathodic substrate at low pH's such as 2.7 with phosphoric acid as the ionizing acid.
Two U.S. patents dealing with nitrogen-based copolymers and their cathodic electrodeposition are U.S. Pat. Nos. 3,455,806 and 3,458,420, both to Spoor, et al., (1969). Cathodic sulfonium systems are described by Wessling et al. on pages 110-127 of "Electrodeposition of Coatings," Ed. E. F. Brewer, American Chemical Society (1973).
Electrodeposition processes have been frequently described in the literature. Two useful reviews of the technology are: "Electro-painting Principles and Process Variables," Brower, Metal Finishing, September, 1976, p. 58; and "Coatings Update: Electrocoating," Americus, Pigment and Resin Technology, August, 1976, p. 17. However, neither of these articles nor any of the patents mentioned above suggest means for obtaining cathodically electrodeposited resin coatings with optimally low levels of water and acid retention and high corrosion resistance.